Abstract

Several varieties of linseed oil are commercially available, and they have been used for many applications especially in decorative furniture finishing and as an oil painting medium. The addition of metallic driers to the linseed oil is a common practice, as it improves the drying rate of the paint. Several cases of fires have been reported, often involving rags soaked with linseed oil which have not been disposed of properly, and it is generally agreed that metal salts in the linseed oil play an important role in the spontaneous ignition of the oil. This paper investigates several types of linseed oil sold in the market and their tendency to cause self-heating of cotton which has been impregnated with linseed oil obtained from various sources. Experiments were performed in two reactors; a plug flow reactor for measurement of gaseous oxidation products and in a batch system for the determination of metal composition in the oil. Both plug flow and batch systems employ fifty percent of oxygen and nitrogen mixtures for several hours in each experiment. Emission of gaseous products during the reaction was quantified by micro gas chromatography (μGC), and identified by Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC-MS). Oil samples from batch system were digested using microwave and the metal composition was determined by inductively coupled plasma-optical emission spectrometer (ICP-OES). Boiled linseed oil is the most reactive oil studied, followed by raw linseed oil and refined linseed oil which display similar reactivity, while the least reactive is stand linseed oil. ICP analysis confirmed the presence of cobalt in the boiled linseed oil, which enhances the rate of oxidation reaction. For boiled linseed oil, the cobalt in the oil enhances the decomposition of peroxide resulting in the formation of various radicals which oxidize the oil and the cotton wool, therefore the emission of gaseous products is higher during oxidation on the cotton wool support compared to glass wool and thus boiled linseed oil has the highest tendency to self heat, especially if soaked on the cotton or rags. A kinetic model of peroxide formation and decomposition has been developed based on the experimental data. The reaction rate constant of the decomposition of peroxide has been found to be higher in the reaction using cobalt compared to that of without cobalt, confirming the role of cobalt in catalysing the peroxide decomposition reaction. Abstraction reactions by the radicals during oxidation lead to subsequent reactions which are generally very exothermic. Accumulation of sufficient heat, can ultimately lead to the temperature reaching the ignition point of the oil and causing the self-heating and even ignition of the oil.